like Don O says, it could be carbon.
I just cut open a Hobbit muffler and there's a very small tube in there, like 1/2 inch inner diameter. _All_ the exhaust must go through that tube. This bike had 5,000 miles on it. That tube was clogged so bad it had a hole the size of a grain of rice. The owner had poked holes in the bottom of the muffler, no doubt because it ran like crap..